help finding vacuum leak... - FFCobra.com

Mark_VA · 05-02-2009, 08:54 PM

There's been a lot of discussion on how to find vacuum leaks, but none of the methods seem to work for me. Help please!

I have a 331 EFI with '93 cobra intake. Currently I'm running the SPOUT out, timing set to 12deg BTDC, IAC plugged in, TPS at 0.96V, new fuel regulator, new fuel pump, stock 19lb injectors, EGR delete, and stock MAF. The idle is fluctuating between ~650 and ~950RPM. A vacuum gauge plugged into a port in the back of the intake reads ~6in. and 10in. Hg, respectively. That seems like a sizable vacuum leak, correct?

I've done both the brake cleaner test and propane torch test, but I can't hear any change in the engine response. What exactly should I be looking/listening for? I've checked all around the intake runners, fuel injectors, uppper/lower intake plenum interface, and around all the sensors.

Any thoughts or suggestions would be greatly appreciated.

shark454 · 05-02-2009, 09:16 PM

The leak is causing the idle to fluctuate. ( from lean to rich and back ). I believe the propane method will cause the engine to race a little, due to the extra fuel. So when you are at the leak, the engine idle should race a bit.

I sometimes use WD 40. Spray it at suspect leak. If the oil gets sucked into the leak, the engine idle will smooth out. Sometimes you can see the liquid actually being sucked into the leak. Try minimizing the vacuum lines. Don't forget the base of the carburator (over tightening a carb causes the base to warp) and the intake manifold edges.

Hope this helps!

nc1mike · 05-02-2009, 11:00 PM

You may never find it. The leak could be in the lifter valley under the intake. If you suspect the intake gasket is leaking, change the intake gasket.
A vacuum leak usually is indicated by a steady reading on the vacuum gauge. Is the change between 6"-10"hg sharp and even or slow and floating.
Here's why I ask if the reading floats that is usually mixture problems. Check PCV etc.
If the movement is quick and rhythmic then it usually mechanical valve spring, sticky valve could even be a plug misfiring? I'd have to see the gauge to know for sure..

Michael S.

Mark_VA · 05-02-2009, 11:03 PM

Thanks for the reply, Shark. I'll try the tests again and listen for the idle to smooth out. I do suspect that the intake may be leaking at the corners between the intake gasket and the RTV seal on the front/rear. I've minimized the vacuum line runs as much as possible.

Anyone else know of any common places for leaks to show up that I should check?

Mark_VA · 05-02-2009, 11:06 PM

Quote:

Originally Posted by nc1mike

A vacuum leak usually is indicated by a steady reading on the vacuum gauge. Is the change between 6"-10"hg sharp and even or slow and floating.

Michael S.

the vacuum reading is going up and down, in step, as the RPM fluctuates.

nc1mike · 05-02-2009, 11:23 PM

If you mean the RPM is low at 6" and higher at 10" kind of rolling or hunting. That has something to do with the idle mixture, it could be a vacuum leak. Now the corner of the intake where the silicone make the seal to the block, if it leaks there that won't be a vacuum leak. That only leads to lifter valley and is there to stop oil leaks.

Mark_VA · 05-02-2009, 11:32 PM

Quote:

Originally Posted by nc1mike

If you mean the RPM is low at 6" and higher at 10" kind of rolling or hunting.

Thats exaclty whats happening. It idles up around 900 or so, then dips down and seems to hesitate a bit at 600-650, then comes back up to around 900. the vacuum gauge goes up and down with the idle speed. Just seems like 10"Hg is really low and the leak should be obvious. what is normal? 13-16"Hg?

nc1mike · 05-03-2009, 01:18 AM

Mark, An "E" cam should idle around 16". Have you been on the road and this happened or is this a new build? It almost sounds like it's running out of gas. I'm going to throw out some suggestions since I don't know if this is a fresh build or a veteran car with a new issue. What happens when you plug or cap off the PCV?
Your description and your sig line don't match. 19 or 24lb injectors? Is the MAF calibrated to match? Have you verified the firing order very carefully?
Anything that causes the idle rpm to fall will also cause the vacuum to fall as it matches RPM.

Michael S.

Pb Foot · 05-03-2009, 01:52 AM

If it is a vac leak, you could also try looking for it around the 19# injectors. If the O-Ring goes bad on some of them, it makes a vac leak VERY hard to find.

Have you cleaned out your IAC? You could remove the IAC and check if it "clicks" with a 9V bat. When I was running EFI, a bad IAC took a while to find.

Have you pulled any codes?

---Mike

StephenP · 05-03-2009, 01:54 AM

Is this a new MAF? My recent install the new (old) MAF was the culprit on the hunting idle. Very dirty - cleaned it and things smoothed out. Took a bit to figure that out. But like the folks above said idle mixture. The computer was being told by the dirty MAF that lots more air was coming in than really was, so the computer was putiing in lots of fuel.

I did have a few leakes. The EGR delete cover (re gasket), a couple of hoses (replaced). I think I have it now at about 15" at idle, I see up to 20 steady state, and down to 0 on hard throttle on.

Good luck, you wil get it. It just may take longer than you want..

Mark_VA · 05-06-2009, 02:20 AM

Michael, Mike, Stephen,
Sorry its taken a few days to update, but your responses really helped get the ideas flowing for tracking this down. It looks like I found the culprit in the vacuum port that comes off the throttle body. The port wasn't sealed where it's inserted into the TB, so a little RTV seemed to do the trick. Now the engine is idling smooth. I'll take a vacuum reading tomorrow to make sure things are good.

nc1mike, I was running 24lb injectors and a 73mm C&L MAF, but the engine was running so rich I was fouling up the plugs. After a little research, it looks like the C&L MAF doesn't have a great reputation. Anyway, I switched back to the stock 19lb injectors and stock MAF. That explains the difference in my sig line. Looks like a Pro-M MAF may be in my future!

I found the leak by trying a different method. I thought it might help to use something a little more volatile to hear a significant difference in the engine RPM. I couldn't get good results with the brake cleaner or propane. But, I'm also safety minded, so I didn't want to spray all kinds of highly flammable materials in the engine bay. So, I took some Q-tips and soaked them in starting fluid. Then, I swabbed the areas with the q-tips to check for leaks. As soon as I passed over the TB vacuum port, the engine shot up in RPM, so I knew I had found a leak. This method made a lot less mess than the brake cleaner, and seemed to get better results than the propane method. Thought I'd share.

Thanks, again, for the responses!!

nc1mike · 05-06-2009, 02:41 AM

Good find and you didn't have to pull the intake. Double Bonus points.